Zvulun Elazar

Biological membranes provide a resilient framework for cellular structure and stability. Disrupting its integrity may result in irreparable damage, altering cellular homeostasis and ultimately leading to cell death. ATG9A, a transmembrane protein, has recently been implicated in plasma membrane repair. However, its role in the process and the mechanism by which it is targeted to the plasma membrane upon damage are unclear. We show here that glycosylation of ATG9A is essential for its membrane repair activity. This has been corroborated by using different mutant cells that are defective in their ability to process proteoglycan in the Golgi complex. Specifically, sialylation of the sugar moiety appears vital for plasma membrane repair activity. Additionally, we provide evidence indicating that ATG9A is targeted to the plasma membrane through interaction with the endosomal sorting complex required for transport complex. Finally, we found that ATG9A lipid scramblase activity and the lipid transfer protein VPS13A are needed for efficient membrane repair.

Gene amplification in the form of extrachromosomal DNA (ecDNA) is a frequent driver in multiple cancer types. As ecDNA lack centromeres, their mitotic segregation does not follow traditional inheritance principles. However, the mechanisms that govern ecDNA fate following mitosis remain unclear. We found that ecDNA undergo numerical and structural optimization under increased selective pressure, with mitotic chromosomal tethering, or detachment, dictating ecDNA fate. When tethered, ecDNA aggregates promote uneven distribution into the newly formed daughter cells, thereby driving inter-cellular numerical heterogeneity and rapid increase of amplification under selective pressure. Mitotically detached ecDNA frequently encapsulate within micronuclei of variable size and content that appear to be highly fragile. Strikingly, ecDNA enclosed in very small micronuclei, which we term nanonuclei, are being actively degraded through autophagy. Together with ongoing structural rearrangements, nanonuclear ecDNA degradation promotes their structural evolution, which facilitates cancer cell adaptation. Our work highlights ecDNA aggregation, micronucleation, and degradation, as pivotal events in directing cancer genome evolution trajectories.

Yeast genetics has the power for thorough investigation of complex systems in molecular cell biology. Here, we present GravyTrain, a comprehensive repository of constructs for genomic modifications, including gene deletions or tagging with fusion tags for robust protein characterization and manipulation. The modular cloning scheme employed by GravyTrain allows shuffling of elements between constructs and potentially the application of included protein tags to research beyond yeast. The experimental potential of GravyTrain is demonstrated by the de novo construction of a library of strains for studying autophagy in yeast.

HSAN9 is a rare progressive neurodegenerative disease in children linked to bi-allelic loss-of-function mutations in the TECPR2 gene. TECPR2 is a multi-domain protein harboring N-terminal WD repeats and C-terminal TECPR repeats, followed by a functional LIR motif that serves in autolysosomal targeting. Here, we show that the lack of TECPR2 leads to impairment of mitophagy that can be recovered by the expression of its C-terminal domain. Accordingly, we uncover severe mitochondrial dysfunction and accumulation of mitochondrial content in primary fibroblasts derived from an HSAN9 patient, and in embryonic fibroblasts and dorsal root ganglia derived from an HSAN9 mouse model. Strikingly, these mitochondrial defects are mediated by a mitochondrial stress through activation of the integrated stress response (ISR), whereas mitochondrial function is recovered by pharmaceutical or genetic suppression of ISR. Our findings provide a new link between mitophagy and ISR in mitochondrial homeostasis during neurodegeneration.

Atg8 proteins play a crucial role in autophagy. There is a single Atg8 isoform in yeast, while mammals have up to seven homologs categorized into LC3s and GABARAPs. The GABARAP subfamily consists of GABARAP, GABARAPL1, and GABARAPL2/GATE16, implicated in various stages along the pathway. However, the intricacies among GABARAP proteins are complex and require a more precise delineation. Here, we introduce a new cellular platform to study autophagy using CRISPR/Cas9-mediated tagging of endogenous genes of the GABARAP subfamily with different fluorescent proteins. This platform allows robust examination of autophagy by flow cytometry of cell populations and monitoring of GABARAP homologs at single-cell resolution using fluorescence microscopy. Strikingly, the simultaneous labeling of the different endogenous GABARAPs allows the identification and isolation of autophagosomes differentially marked by these proteins. Using this system, we found that the different GABARAPs are associated with different autophagosomes. We argue that this new cellular platform will be crucial in studying the unique roles of individual GABARAP proteins in autophagy and other putative cellular processes.